Current day phosphate coating solutions are dilute solutions of phosphoric acid and other chemicals which are applied to the surface of metals; the surface of the metal reacts with the solution and forms an integral layer (on the surface of the metal) of substantially insoluble amorphous or crystalline phosphate coating. Crystalline coatings result if zinc or similar divalent metals other than ferrous ion are present. Depending on the characteristics of the coating, it may function to enhance corrosion resistance, wear resistance or electrical resistance; as a base for the application of a second coating (e.g., paint); or as a vehicle to retain a lubricant on the coated surface preparatory to cold forming.
Certain of these solutions have achieved widespread commercial use. Such solutions typically include phosphate ions, zinc and/or manganese ions and typically one or more of the following ions: nickel, cobalt, copper, nitrate, nitrite, chlorate, fluoborate or silicofluoride. The art has been able to form phosphate coatings since about 1917, and there have been successive discoveries of the effects of the nitrate, copper, nickel, fluoborate, and silicofluoride ions on the coating ability of such solutions made through the years. Presently, metal surfaces are typically provided with a phosphate coating by being treated in the following process sequence: (1) cleaning; (2) conditioning; (3) phosphating; and (4) post-treating. Rinses are usually employed between steps to avoid drag-in to the next stage. Such processes and solutions for forming conversion coatings on metal surfaces are well known and have been described, for example, in "Metal Handbook", Volume II, 8th Edition, pages 529-693 (1972), the contents of which are specifically incorporated herein by reference. Despite these advances, the best present day formulations are troublesome in certain respects and accordingly there is a continuing demand for still further improvements in the compositions and processes. In particular, known processes are difficult to control, form undesirably high coating weights, cause the formation of scale on processing equipment, and require replenishment as two or more separate additions. Such improvements are still being sought particularly in ferrous-, zinc- and aluminum-based substrates employed in automotive paint-base applications.
It is recognized that there are distinct types of crystal morphologies that can be produced on ferrous surfaces by zinc phosphate coating solutions. The crystalline structure may be platelet, columnar or nodular in form when examined with an electron microscope. The platelet structure resembles relatively large plates or flakes of crystalline material. The columnar configuration resembles smaller column shaped crystals, and the nodular configuration resembles uniformly disposed small nodular shaped crystals. The latter two configurations are generally preferred for paint base applications on ferrous surfaces because they achieve equivalent or better performance with respect to paint adhesion and physical tests compared to the platelet configuration. The columnar and nodular coatings are also lower in coating weight which is beneficial where cathodic electropainting is to be employed.
It has been recognized that the columnar and nodular forms are obtained by employing baths with zinc concentrations at relatively low levels. See for example U.S. Pat. No. 4,330,345 and U.S. Pat. No. 4,419,199. One of the problems encountered is that if the zinc levels increase in the bath, for example due to dissolution of zinc from galvanized parts or due to process control problems, the form of the coating will rapidly change to the less desirable platelet morphology. It would therefore be desirable to have a coating bath and process which broadens the range of acceptable zinc levels which will still form the desired columnar and/or nodular coatings.
Another problem inherent in other phosphating processes is the formation of scale on heat exchangers and on the piping and related equipment used to circulate and/or apply the solution to the parts which must be periodically removed to maintain both heating efficiency and/or coating quality. Moreover, many processes currently employed use nitrite as accelerator and therefore require the replenishment of the phosphating solutions from two separate replenishing packages because nitrite would decompose in the concentrated acidic replenisher. Finally, modern phosphating processes must be capable of successfully forming a phosphate coating on galvanized and aluminum surfaces in addition to ferrous surfaces.
The present invention solves the foregoing problems in producing the desired paint-base coating through the inclusion of a hydroxylamine agent in the zinc phosphate solution.
Hydroxylamine agents have been disclosed for use in certain zinc phosphate solutions. For example, U.S. Pat. No. 2,743,204, issued Apr. 24, 1956 to Russell discloses a metal (iron, zinc and manganese) phosphate coating solution having a pH of about 1.9 to about 3.5. The patentee states that the coating weight resulting from such conventional aqueous acidic phosphate solutions may be increased by the addition of small quantities of certain organic chelating agents. Hydroxylamine is recited as one of many specific oxidizing agents which can be used in such coatings. This patent is directed only to heavy phosphate coatings desirable for base corrosion resistance or cold forming purposes and does not contemplate paint base applications. The specific levels of zinc and hydroxylamine exemplified yield platelet morphology and the inventor does not recognize the potential benefits of the hydroxylamine to produce columnar and/or nodular coatings for paint base applications.
U.S. Pat. No. 2,298,280, issued Oct., 1942 to Clifford, et al. discloses the use of hydroxylamine in a coating acid phosphate solution to accelerate the coating action of the solution. Again, the specific levels of zinc and hydroxylamine exemplified yield platelet morphology and the inventor does not recognize the potential benefits of the hydroxylamine to produce columnar and/or nodular coatings.
U.S. Pat. 4,149,909, issued Apr. 17, 1979 to Hamilton discloses iron phosphate coating processes for applying a moderate coating weight on ferrous metal surfaces by spraying or dipping in the solution. The process employs a combination accelerator comprising hydroxylamine sulfate and an oxidizing agent such as a chlorate or a bromate. The resulting amorphous coatings do not relate to the crystalline coatings of a zinc phosphate system.
U.S. Pat. No. 4,003,761, issued Jan. 18, 1977 to Gotta, et al. discloses a process for applying a phosphate coating to a ferric surface by spraying. The patentee states that an improvement in the production of phosphate coatings by spraying acid solutions based on alkali metal and/or ammonium orthophosphate is accomplished by the addition of 0.05 to 1 grams per liter of a short-chain alklolamine and from about 0.01 to 1.5 grams per liter of a non-ionic wetting agent. The patentee further states that oxidizing or reducing agent accelerators can be employed; he includes hydroxylamine salts as one of the many groups of such compositions that may be employed. It is stated that the pH value of the solution is in the range of 4.3 to 6.5, that the duration of treatment for the spray is 0.5 to 5 minutes and that the process can be carried out at temperatures between 40.degree. C. and 95.degree. C., preferably 50.degree. C. to 70.degree. C. Amorphous coatings result from this process.
U.S. Pat. No. 2,702,768, issued Feb. 22, 1955 to Hyans, et al. discloses that the coating provided by "non-coating phosphate" solutions can be improved by employing hydroxylamine in the solution. "Non-coating phosphates" are specified as being alkali metal phosphates such as sodium phosphate and potassium phosphate, as well as ammonium phosphate. It is suggested that the hydroxylamine be used at a level of 0.1% to 0.5% and at a pH of about 4.2 to 5.8. Again, amorphous coatings are the result.
U.S. Pat. No. 3,615,912, issued Oct. 26, 1971 to Mainz-Kostheim, et al. discloses treating and coating solutions containing alkali- or ammonium-based orthophosphates with hydroxylamine being an optional ingredient. Amorphous coatings result from this process.
U.S. Pat. No. 4,220,486, issued Sept. 2, 1980 to Matusushima, et al. (assigned to Nihon Parkerizing Company, Japan) describes an alkali phosphate conversion coating solution containing stannous ions and fluoride ions and optionally pyrazole compounds, hydroxylamine compounds and hydrazine compounds at a level of 0.2 grams per liter to about 5 grams per liter. This process does not produce a crystalline zinc phosphate coating.
A treatment solution that attempts to control the crystal morphology is disclosed in European patent application No. 0,175,606, based upon French patent application FR No. 8412878, filed Aug. 16, 1984. The use of a hydroxylamine agent is not suggested.
In none of the prior art is there even a general teaching or suggestion that the use of hydroxylamine critically influences crystal morphology in any fashion.